Constant effective voltage power source circuit

ABSTRACT

A constant effective voltage power source circuit utilizing the charging and discharging of a capacitor. It includes an output voltage detection circuit having non-linear elements such as a diode and a constant-voltage diode which impart a parabolic current-voltage characteristic to the power source circuit. The output voltage detection circuit functions to vary the charging current into the capacitor, which is repeatedly charged and discharged to provide an effective output voltage, in accordance with the source voltage detected by it to control the effective output voltage to a constant value.

United States Patent Kawashima 1451 Dec. 26, 1972 [54] CONSTANT EFFECTIVE VOLTAGE 'POWER'SOURCE CIRCUIT [72] Inventor: Yoshichi Kawashinia, Gifu, Japan [73] Assignee: Nippondenso KabushikiKaisha 221 Filed: Nov. 3, 1970 [21] Appl. No.: 86,399

[30] Foreign Application Priority Data Dec. 16, 1969 Japan ..44 1o1056 [52] US. Cl. .....307/297, 307/290, 323/22 T [51] Int. Cl ..G05f 1/40 [58] Field Of Search ..307/297305; 323/22 T [56] References Cited UNITED STATES PATENTS 3,317,820 5 1967 Nylander ..3o7 297 3,262,045 7/1966 Hauclt ..307/297 3,174,096 3/ l 965 Lichowsky 3,124,698 3/1964 Semmer et al .L ..307/297 Primary Examiner-James W. Lawrence Assistant Examiner-Harold A. Dixon Attorney-Cushman, Darby & Cushman [5 7 ABSTRACT A constant effective voltage power source circuit utilizing the charging and discharging of a capacitor. It includes an output voltage detection circuit having non-linear elements such as a diode and a constantvoltage diode which impart a parabolic current-voltage characteristic to the power source circuit. The output voltage detection circuit functions to vary the charging current into. the capacitor, which is repeatedly charged and discharged to provide an effective output voltage, in accordance with the source voltage detected by it to contr'ol the effective output voltage to a constant value.

5 Claims, 4 Drawing Figures 01L CAN uAmTv DETECTOR PATENTEDIJECZB m2 3. 707.835

SHEET 1 UF 2 OIL CAN QUANITI DETECTOR INVENTOR CONSTANT EFFECTIVE VOLTAGE POWER SOURCE CIRCUIT BACKGROUND OF THE INVENTION 1 Field of the Invention This invention relates to a constant effective voltage power source circuit.

2. Description of the Prior Art The typical conventional constant output power source circuit used, for instant, in an automobile oil gauge-employs a bimetal strip. As shown in FIG. 1 of the accompanying drawing, it comprises a bimetal 6 provided with a contact 7 anda heater coil 5. When the bimetal contact 7 is in contact with a fixed contact, the source voltage of a power supply 3 appearing across input terminals 1 and 1 is applied across the heater 5 to heat the bimetal 6. As soon as a predetermined tem' perature of the bimetal is exceeded, the bimetal contact 7 is opened. Subsequently, when the bimetal temperature is reducedto a certain value, the contact 7 is re-closed. The above cycle of events is repeated to maintain the output power constant.

In the aforementioned prior art device using the bimetal 6, however, the heater temperature is affected by the ambient temperature, so that its temperature characteristic is inferior Also, its service life is relatively short, because of the fact that the contact 7 is used. Further, the make-and-break of the contact 7 would give rise to noise in radio broadcasting. Furthermore, the cycle of make-and-break of the contact 7 is relatively long, so that the pointer 10a of the oil gauge oscillates. Moreover, time delay is involved from the closing of switch 4 until the heater 5 is sufficiently warmed up, so that the response characteristic is inferi- SUMMARY OF THE INVENTION It is an object of the invention to overcome the foregoing disadvantages by the provision of a transistorizcd constant output power source circuit.

Another object of the invention is to provide a constant effective voltage power source circuit comprising a transistor circuit to the on-off control current supplied to a load, a capacitor charged with load current, a switching circuit to on-off control said transistor circuit in accordance with the terminal voltage across said capacitor and an output voltage detection circuit including non-linear elements connected in the charging path of said capacitor.

According to the invention, it is possible to provide a square-wave constant effective voltage output and minimize the power consumption of output transistors. Thus, excellent effects can be featured in that inexpensive transistors consuming low power may be used and that the radiator plate for the output transistors may be made extremely small in size, which is very advantageous for the integration of the power source circuit.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a circuit diagram showing the conventional constant effective voltage power source circuit used in the oil gauge.

- FIG. 2 is a circuit diagram showing a constant effective voltage power source circuit used in an oil gauge embodying the invention.

FIG. 3 shows the waveform of the output voltage of the constant effective voltage power source circuit according to the invention.

FIG. 4 is a graph showing a current-voltage characteristic approximating circuit according to the inventron.

DESCRIPTION OF THE PREFERRED EMBODIMENT The invention will now be described in connection with a preferred embodiment thereof shown in FIG. 2. In the Figure, like parts as those in FIG. 1 are designated by identical reference numerals. Numeral 11 designates a variable resistor whose resistance is varied in accordance with, for instance, the quantity of oil in an oil tank. Numeral 12 designates a resistor, numeral 13 a transistor, and numeral 14 a constant-voltage diode. These three elements constitute a compensating circuit to compensate for source voltage variations. Numerals 16 and 17 designate transistors in Darlington connection to on-off control the load current. Numeral 18 and 19 designate transistors in Darlington connection to on-off control the transistors 16 and 17. The former transistors are connected through a resistor l5 across input terminals 1 and 1'. Numerals 24 and 26 designate transistors constituting a Schmitt circuit to on-off control the transistors 18 and 19 in accordance with the terminal voltage across a capacitor 38. Numeral 27 designates a constant-voltage diode, numerals 20, 23, 25 and 39 resistors, and numerals 21 and 22 diodes. Enclosed within a dashed rectangle A is an output voltage detection circuit including non-linear elements. It comprises resistors 28, 37, 34 and 36, diodes 29, 30, 31, 32 and 33 and a constant-voltage diode 35.

In the operation of the circuit construction described above according to the invention, when the terminal voltage across the capacitor 38 is lower than the zener voltage of the constant-voltage diode 27, the transistor 26 is off, the transistor 24 is on", the transistors 19 and 18 are off, and the transistors 16 and 17 are on". In this state, power is supplied to the oil gauge 10, while at the same time the terminal voltage across the capacitor 38 is increasing. As soon as the terminal voltage across the capacitor 38 exceeds the zener voltage of the constant-voltage diode 27, the transistor 26 is triggered, the transistor 24 is cut off, the transistors 18 and 19 are triggered, and the transistors 16 and 17 are cut off. The cyclic mode of operation described above provides an output voltage waveform as shown in FIG. 3. The effective value V of the output voltage shown in FIG. 3 is given as:

eg e V 1 l 2) (I) where V,,, is the source voltage, t is the charging time interval from the instant of cutting-off of the transistor 26 till the instant at which the terminal voltage across the capacitor 38 becomes sufficient to trigger the transistor 26 through the constant-voltage diode 27, and is the discharging time interval from the triggering till the cutting-off of the transistor 26 during which the terminal voltage across the capacitor 38 continues to decrease. 7

It will be seen that the circuit shown in FIG. 2 operates to provide for a constant value of the terminal voltage across the capacitor 38. Thus, if the constant voltage detection circuit indicated at A offered a constant resistance, the operation would be such that the average value of the current through such constant resistance, that is, the average value of the output voltage, is controlled to the constant. Stated in an equation;

V,, X t,/(t t a constant.

Thus, the effective voltage will not be constant. In order that the effective value V of the output voltage be constant, the right side of equation (I), and hence its square, should be constant. That is,

V82 X t /(t KKK, being a constant). Solving this equation for t i 1 2/( i) Since which is the discharging time interval for the capacitor 38 to be discharged, is solely determined by the resistor 28 in the discharging circuit, it is desired that I, will vary with change in V such that equation (2) is satisfied. Since I, is inversely proportional to the resistance R of the output voltage detection circuit A constituting the charging path, through which the capacitor 38 is charged, the circuit parameters of the output voltage detection circuit may be appropriately selected in such a manner that the aforesaid resistance R can be approximated by a second order function of the source voltage V as expressed by an equation;

R K2 V32 (K2 being a constant), so that the effective voltage V, may be made constant. In this manner, a substantially parabolic current-voltage characteristic as shown in FIG. 4 may be obtained. If the source voltage V of the power supply 3 of FIG. 2 is low, the capacitor 38 is charged only through the resistor 28 (corresponding to the slope of plot I in FIG. 4). If V exceeds a predetermined level, the diode 29 is triggered to cause additional charging current through the resistor 37 (corresponding to the slope of plot II in FIG. 4). If V exceeds a higher predetermined level, the diodes 30 to 33 are triggered to cause further additional charging current through the resistor 34 (corresponding to the slope of plot III in FIG. 4). Finally, if V,, exceeds a still higher predetermined level, the constant-voltage diode 35 is triggered to cause a still further additional charging current through the resistor 36 (corresponding to the slope of plot IV in FIG. 4). With this current-voltage characteristic the effective voltage V is held constant.

The preceding embodiment has concerned an automobile oil gauge as the load. It is, however, to be understood that the oil gauge is by no means limitative, but the invention may also be applied to an engine temperature gauge to detect and indicate the temperature of an engine on an automobile. In this case, the variable resistor 11 in the preceding embodiment may be replaced with a heat-sensitive element such as thermistor or posistor. Also, the invention is not limited to the automobile applications, but various other applications of the invention are possible.

Iclaim:

1. A circuit for supplying constant power to a load comprising:

means for receiving an input voltage,

electronic switch means connecting said receiving means to said load and having a conducting and non-conductive condition,

diode to conduct through said second resistor, and

electronic control means connected to said capacitor and to said switch means for causing said switch means to shift from a first to a second condition when the charge on said capacitor exceeds a predetermined value and thereafter to shift back to said first condition so that the power supplied to said load remains constant and for causing said capacitor to discharge while said switch means is in said second condition.

2. A circuit as in claim 1 wherein said electronic switch means includes a pair of Darlington connected transistors.

3. A circuit as in claim 1 wherein said non'linear resistive means further includes a third resistive branch connected in parallel with said first resistor comprising a plurality of diodes serially connected with said diode and a third resistor serially connected with said plurality of diodes and a fourth branch serially connected with said first resistor comprising a zener diode serially connected with said plurality of diodes and a fourth resistor serially connected with said zener diode.

4. A circuit for supplying constant power to a load comprising:

means for receiving an input voltage,

electronic switch means connecting said receiving means to said load and having a conducting and non-conductive condition,

a capacitor,

non-linear resistive means connecting said capacitor to said load so that the rate of charging of said capacitor varies as a function of the voltage at said load, and

electronic control means connected to said capacitor and to said switch means for causing said switch means to shift from a first to a second condition when the charge on said capacitor exceeds a predetermined value and thereafter to shift back to said condition so that the power supplied to said load remains constant and for causing said capacitor to discharge while said switch means is in said second condition, including a Schmidt trigger circuit, a zener diode connecting said capacitor to said Schmidt trigger circuit for causing said Schmidt trigger to produce a pulse when the charge on said capacitor exceeds said predetermined value, and a pair of Darlington connected transistors for causing said electronic switch means to shift from said non-conductive to said conductive condition when said Schmidt trigger circuit produces said pulse.

5. A circuit for supplying constant power to a load comprising:

a load including an oil gauge and resistive means serially connected with said gauge having a resistance which varies as a function of the quantity of oil in a can,

run:

and to said switch means for causing said switch means to shift from a first to a second condition when the charge on said capacitor exceeds a predetermined value and thereafter to shift back to said first condition so that the power supplied-to said load remains constant and for causing said capacitor to discharge while said switch means is in said second condition. 

1. A circuit for supplying constant power to a load comprising: means for receiving an input voltage, electronic switch means connecting said receiving means to said load and having a conducting and non-conductive condition, a capacitor, non-linear resistive means connecting said capacitor to said load so that the rate of charging of said capacitor varies as a function of the voltage at said load, including a first resistor, a second resistive branch connected in parallel with said first resistor and comprising a serially connected second resistor and diode so that the effective resistance between said load and said capacitor is reduced at a predetermined load voltage which causes said diode to conduct through said second resistor, and electronic control means connected to said capacitor and to said switch means for causing said switch means to shift from a first to a second condition when the charge on said capacitor exceeds a predetermined value and thereafter to shift back to said first condition so that the power supplied to said load remains constant and foR causing said capacitor to discharge while said switch means is in said second condition.
 2. A circuit as in claim 1 wherein said electronic switch means includes a pair of Darlington connected transistors.
 3. A circuit as in claim 1 wherein said non-linear resistive means further includes a third resistive branch connected in parallel with said first resistor comprising a plurality of diodes serially connected with said diode and a third resistor serially connected with said plurality of diodes and a fourth branch serially connected with said first resistor comprising a zener diode serially connected with said plurality of diodes and a fourth resistor serially connected with said zener diode.
 4. A circuit for supplying constant power to a load comprising: means for receiving an input voltage, electronic switch means connecting said receiving means to said load and having a conducting and non-conductive condition, a capacitor, non-linear resistive means connecting said capacitor to said load so that the rate of charging of said capacitor varies as a function of the voltage at said load, and electronic control means connected to said capacitor and to said switch means for causing said switch means to shift from a first to a second condition when the charge on said capacitor exceeds a predetermined value and thereafter to shift back to said condition so that the power supplied to said load remains constant and for causing said capacitor to discharge while said switch means is in said second condition, including a Schmidt trigger circuit, a zener diode connecting said capacitor to said Schmidt trigger circuit for causing said Schmidt trigger to produce a pulse when the charge on said capacitor exceeds said predetermined value, and a pair of Darlington connected transistors for causing said electronic switch means to shift from said non-conductive to said conductive condition when said Schmidt trigger circuit produces said pulse.
 5. A circuit for supplying constant power to a load comprising: a load including an oil gauge and resistive means serially connected with said gauge having a resistance which varies as a function of the quantity of oil in a can, means for receiving an input voltage, electronic switch means connecting said receiving means to said load and having a conductive and non-conductive condition, a capacitor, non-linear resistive means connecting said capacitor to said load so that the rate of charging of said capacitor varies as a function of the voltage at said load, and electronic control means connected to said capacitor and to said switch means for causing said switch means to shift from a first to a second condition when the charge on said capacitor exceeds a predetermined value and thereafter to shift back to said first condition so that the power supplied to said load remains constant and for causing said capacitor to discharge while said switch means is in said second condition. 